Showing papers by "Arthur D. Richmond published in 1991"

Ionospheric convection response to changing IMF direction

Abstract: By combining ground-based and satellite-based measurements of ionospheric electric fields, conductivities and magnetic perturbations, it was possible to examine the characteristics of instantaneous, ionospheric convection patterns associated with changing directions of the interplanetary magnetic field (IMF). In response to a rapid southward-to-northward turning of the IMF on July 23, 1983, the ionospheric convection reconfigured over a period of 40 minutes. The configuration changed from a conventional two-cell pattern to a contracted four-cell pattern, with reversed convection cells in the high-latitude dayside, associated with a strong potential drop of about 75 kV. Later, in response to a gradual rotation of the IMF from the +Z through the -Y toward the -Z direction, the nightside cells disappeared and the dawn cell in the reversed pair wrapped around and displaced the dusk cell until a conventional two-cell pattern was reestablished, largely in accord with the qualitative model of Crooker (1988). The results suggest that multiple cells can arise as a result of strong southward to northward transitions in the IMF. They appear to persist for sometime thereafter.

Radar interferometry technique and anisotropy of the echo power distribution: First results

Abstract: The theory of radar interferometry developed in the companion paper [ Van Baelen and Richmond, this issue] is applied to a set of measurements obtained with the middle and upper atmosphere (MU) radar in order to demonstrate the technique and to test the theory. The variations with respect to Doppler frequency of the coherencies and phases of the signals cross-correlated between receiver pairs depend on the amplitude and direction of the wind in the manner predicted by the theory. The horizontal wind derived by the interferometric technique agrees quite well with the so-called apparent velocity obtained by cross-correlating signals in the time domain from different receiver pairs, and under our experimental conditions the latter estimate has been determined to be a good representation of the true wind, in spite of slight tendencies to overestimate the wind amplitude. The vertical wind derived by the interferometric technique shows significant differences with the Doppler wind obtained from a vertically pointing antenna, but these differences disappear when the Doppler wind is corrected for the off-zenith distribution of the received power. An examination of the mean angle-of-arrival of the received signal reveals that there is a tendency for the angle to approach, though not attain, perpendicularity with the three-dimensional wind vector, and perhaps also to tend to be perpendicular to isentropes in the crosswind direction. A postset beam steering analysis of several examples of data illustrates the behavior of the angular distribution of received power in time and space. When the signal-to-noise ratio is large, there is a tendency for the power to be fairly isotropic with respect to zenith angle, but highly distorted patterns can be encountered when the signal is weaker. There appears to be a tendency for the patterns to vary relatively slowly over time, but strong variations in altitude are observed in passing from one 300-m range gate to the next. The direction of most intense received power does not necessarily correspond to the direction of mean angle-of-arrival of the signal as determined by interferometry.

Radar interferometry technique: Three-dimensional wind measurement theory

Abstract: Generalizing radar interferometry to three-dimensional space, we present a model which predicts the variation with respect to frequency of the phase of the cross-spectrum obtained between two separate antennas. The slope of this variation is proportional to the cosine of the angle between the horizontal wind and the interferometer baseline, and inversely proportional to the horizontal wind amplitude. The zero-frequency phase depends on the horizontal wind speed and direction, on the vertical wind velocity, and on the anisotropy of the backscattered power with respect to the zenith. The model also allows us to predict qualitatively the effects of specularity, beam shape, and random turbulence on the phase versus frequency variation. Assuming a spaced receiver setup with three baselines rotated by 120°, we present an implementation scheme of the radar interferometry technique which yields a three-dimensional wind estimate and a parameter characterizing the anisotropy of the echo power distribution. Finally, we argue that the radar interferometry horizontal wind measurements will suffer from limitations similar to those of the so-called apparent velocity as both techniques are related by Fourier transform, and that the vertical wind component will be unbiaised by anisotropy in the backscattered power.

The Ionospheric Wind Dynamo

Abstract: This paper presents a brief review of recent work concerning the global Ionospheric wind dynamo, with a view toward helping to define some principal directions for future research. Remaining problem areas are identified as: more accurate determination of the distributions of ionospheric conductivities, especially in the highly variable F region; determination of the distributions of winds in the E region under all seasonal conditions, especially the tidal components; improved understanding of the mutual coupling effects among variations in conductivities, thermospheric winds, and electric fields and currents; and quantitative understanding of the mutual coupling processes acting between the magnetospheric and ionospheric dynamos and the changes in the ionospheric dynamo occurring during magnetic storms.

Auroral effects on midlatitude semidiurnal tides

Abstract: The effect of auroral activity on mid-latitude semidiurnal tides was investigated using simulations from the NCAR Thermosphere/Ionosphere General Circulation Model (TIGCM). Model runs were made for solar cycle minimum equinox conditions for four levels of geomagnetic activity parameterized by the total hemispheric power index and the cross polar cap potential drop. Simulations at 42.5°N (gg) predicted that the upper thermosphere semidiurnal winds and temperatures generally increase with increasing geomagnetic activity, while the lower thermosphere fields were relatively insensitive to the level of auroral forcing in the model. The modelled semidiurnal mid-latitude tidal response was determined by the magnitude and phasing of the waves generated by in situ solar forcing and the auroral momentum and energy sources, in conjunction with those propagating up from the lower atmosphere. The predicted sensitivity of the model tides to the level of geomagnetic activity may contribute to the observed tidal variability at mid latitudes. Successful modelling of observations will require careful specification of the high-latitude energy and momentum sources.